Optical instruments based on light sources that exhibit variations in the angle at which the light beam leaves the light source pose problems when position-sensitive detectors are used to monitor the light beam or the light leaving a sample chamber. For example, tunable lasers often exhibit significant pointing errors. Tunable lasers also exhibit power fluctuations. A correction for the power fluctuations can be provided by a detector that monitors the output of the laser. However, detectors, especially in the mid and far infrared wavelength range, are position sensitive. If the beam shifts position on the detector during the measurements, the signal from the detector will vary even if the light intensity does not change. The pointing errors typically result in such shifts in position, and hence, pose challenges when the detector is used to reduce power fluctuation noise.